1. Field of the Invention
This invention relates to a method and apparatus for measuring fluid currents, particularly water currents, in order to obtain surface or bulk current distributions. More particularly, this invention relates to the use of accoustic transmissions in fluids, with eikonal approximations and tomographic techniques used to derive current measurement from the phase information of acoustic signals.
2. Description of the Related Art
There exists a need for on-site determination of water currents. Knowledge of water currents is important in monitoring and predicting conditions in various marine ecosystems, including fisheries. Such knowledge can also be beneficial in determining the transport of sediment and pollutants. Knowledge of pollutant transport can help to mitigate damage to fisheries and other wildlife habitats.
Previous methods for determining currents have included flow meters and high-frequency radar. Flow meters can provide on-site measurements, but not at the water's surface where wind effects are dominant. High-frequency radar can be used to measure surface currents, but the range resolution of such radar does not provide details at scales less than a kilometer.
Acoustic waves have been used to detect and map underwater objects such as submarines and ocean floor profiles. It is also known that a sound wave traveling in a fluid contains information about physical properties of the fluid itself. Previous acoustic tomographic techniques have used an inversion method or inverse calculation from the travel time of the acoustic wave to determine properties of the fluid such as temperature, salinity, and density. Other techniques have concentrated on the intensity and amplitude of the acoustic wave as sources of information.